Electronic circuits produce electromagnetic interference (EMI) and radio frequency interference (RFI). The noise can interfere with the operation of televisions, radios, telephones and similar equipment. The electrical noise can conduct through the power lines or radiate through power line or through output leads.
An electronic ballast or any other electronic circuit for powering fluorescent lights is affected by EMI/RFI noise and also produce EMI/RFI noise. A filter is therefore needed to reduce the EMI/RFI noise produced by the ballasts and to filter the EMI/RFI noise from the lines powering the ballasts.
Additionally, a filter for an electronic ballast should filter any power line transients, as well as decrease THD (total harmonic distortion).
There are two types of EMI/RFI noise. Differential noise is interference generated between the power line and the return line. Common mode noise is interference between both the power line and the return line and ground.
In some electronic ballasts, an inductor is placed in series in the power line to eliminate differential noise. Elimination of common mode noise is done by way of two inductors wound on the same core. One of the two inductors is in series with the power line and the second inductor in series with the return line.
Use of one component for the differential inductor and another component for the common mode inductor increases the cost of the ballast. The cost is high. There is difficulty in manufacturing the magnetics as well as assembly of the magnetics into the circuit board. Additional space is required on the board.